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Waterfurnace wiring

Hi,
I have a 4 ton Envision Waterfurnace with backup elect. heat.Waterfurnace's own schematic shows the ECM fan motor is powered along with main compressor power "without" the backup elect. heat but powers the ECM fan "with" the backup main power when backup heat is installed.
I don't see a problem with that except recently (during this ice storm and power failure) when using my backup generator the furnace back up heat would require more power than my generator would support. The generator is rated far more than the furnace needs without the elect. backup (yes, even when starting). but turning the breakers off for the electric backup of course kills the furnace as current wiring also kills ECM blower power.
The system worked great this past summer when I used it to power the furnace in cooling mode.
Why do they recommend powering the ECM blower with backup heat power?
Thank you for any help, Mike

As stated in the header, this forum is primarily a welcome area and not for asking technical questions. Your question would be better posed in the Residential HVAC forum down the page a bit in The Arpa Zone section.

We suggest powering the blower motor with the auxiliary heat circuit so that the unit will still provide heat in the event that the compressor would trip a breaker. The auxiliary heat circuit powers the microprocessor, blower motor, thermostat, and electric elements. Without this circuit being powered, you have no controls and the unit will not run.